Melamine is generally produced by heating molten urea in an ammonia-fluidized catalytic reactor above 350.degree. C. Known as the Stamicarbon process, the fluidized catalytic preparation of melamine from urea is described in U. S. Pat. Nos. 3,598,818 to Krekels and 3,682,911 to Kaasenbrood et al. Typically, the melamine in the reaction effluent is quenched with an aqueous liquid to extract the melamine. Carbon dioxide and water vapor remaining in the reaction effluent are absorbed by an ammonia absorbent to produce an aqueous ammonium carbamate solution useful as a reactant for urea production. Excess ammonia from the absorption step is recycled to the melamine reactor as a catalyst fluidizing gas and to suppress by-product reactions. Currently, fluidized bed melamine reactors are operated at a relatively lower pressure (less than about 1.0 MPa) to permit ammonia recovered from the reaction effluent gas to be recycled directly to the melamine reactor.
Several drawbacks to the prior art are evident. The use of lower pressure in the melamine reactor produces an effluent stream having a relatively high water partial pressure following melamine recovery. Upon CO.sub.2 absorption (and carbamate condensation), the carbamate solution produced is generally too dilute for recycling to the urea plant without an intervening concentration step. Further, the pressure in the ammonia recovery is too low to permit economical ammonia condensation without expensive refrigeration. Thus, the use of oxygen passivation for inhibiting corrosion in the quenching and stripping equipment is limited since there is no provision for separating the oxygen from the ammonia recycled to the reactor. In addition, melamine recovery in the Stamicarbon process produces a slurry containing melamine solids. The presence of solids can lead to plugging problems and require the use of solids separation equipment such as cyclones.